This invention relates to belts used in heat fixing of toner. Such belts typically are moved across a heating element while in contact with paper or other media carrying toner to be fixed into such media by fusing. Such belts desirably have excellent characteristics of heat resistance, heat conductivity, strength with flexibility, and low dielectric constant.
Polyimides are attractive in a number of high temperature applications because of their excellent heat resistance and mechanical performance. But like all organic polymers they are inherently heat insulating. Therefore when heat conduction is a desired property, it is common to incorporate inorganic powders such as boron nitride, beryllium oxide, aluminum nitride, silicon carbide, silicon nitride, alumina and silica (in order of decreasing thermal conductivity), that possess high thermal conductivity. In addition, if electrical insulation is also required, the choices are reduced to boron nitride, beryllium oxide, silicon nitride, aluminum nitride and alumina, (in order of increasing dielectric constants).
Among the above mentioned fillers, boron nitride is most widely used because of its combination of high thermal conductivity, low dielectric constant (even at high temperatures), non abrading and non toxic properties. However the addition of boron nitride exponentially degrades the flex fatigue of the polyimide film.
A polyimide that is filled with boron nitride is potentially useful belt fuser. In belt fusers, for which that disclosed in U.S. Pat. No. 6,157,806 is illustrative, toner is fused using a seamless, endless belt which is moved across a ceramic heater. A sheet carrying lose toner in the form of an image is pressed against the belt and heat transmitted by the belt fuses the toner into or onto the sheet. Such a belt is subjected to high temperature and repetitive flexing, while good heat transmission is required. The polyimide layer should be electrically insulative to help prevent electrical short circuits or arcing from the heater. The outer layer, typically of fluoropolymer, should be electrically conductive enough to prevent high voltage from developing on the surface. This is important so as not to attract the toner to the belt.
This invention includes the modification of the surface of boron nitride by oxidizing in air. The oxidation of the boron nitride causes an increase in the flexibility of a polyimide film when compared to a film made with unoxidized boron nitride, at the same filler loading. Oxidized boron nitride is known is the prior art, such as in U.S. Pat. No. 4,406,825 to Pez et al, while a use to improve flexibility is believed novel with respect to this invention.
This invention comprises a fuser belt of polyimide incorporating surface oxidized boron nitride. The resulting enhanced flexibility provides continuing strength without physical damage during use of the belt in a belt fuser. The extent of flexibility enhancement observed is dependent on the degree of oxidation. Therefore, oxidation temperature and time of oxidation are key variables that are used to control the degree of oxidation and thereby the resulting improvement in the flex fatigue.